The subject matter disclosed herein relates to the control of electric motors. More specifically, the subject matter discloses an improved transition between operating modes in a soft starter.
Induction motors are widely used electrical machines. The most basic control of an induction motor utilizes a contactor to alternately connect and disconnect the induction motor to a fixed power supply. Contactor control provides a very inexpensive method for starting and stopping a motor. However, upon connection to the power supply, the motor will accelerate up to its rated speed in an unregulated manner, as quickly as possible, and drawing as much current as necessary from the power supply.
In contrast, high performance control of an induction machine is commonly achieved by using a variable frequency drive. A variable frequency drive can regulate the speed of and control the current in a motor. With the addition of a position sensing device, such as an encoder, the variable frequency drive can also regulate speed and current down to zero speed of the motor. Consequently, variable frequency drives permit controlled acceleration and deceleration of a motor, as well as operation at a wide range of operating speeds. However, variable frequency drives and encoders can add significantly to the complexity and expense of a motor control system.
Soft-starters provide yet another option for connecting motors to a power supply. A soft-starter provides a predefined speed profile for the motor during acceleration and deceleration, limiting the current drawn by the motor. While soft-starters typically do not provide the same level of control afforded by a variable frequency drive, they do reduce the wear on a motor and provide a simple, cost-effective means of connecting the motor to the power supply.
Electric soft-starters typically include a solid-state device connected in series between each phase of the power supply and the motor. The solid-state device is selectively turned on and off for a portion of the electrical cycle, controlling the voltage supplied to the motor. Many methods exist for controlling the solid-state devices in soft-starters. However, it has been found that different control methods perform better under different operating conditions. For example, control methods exist that result in improved operation at higher motor speeds while others provide better operation at lower motor speeds.
However, attempting to execute multiple control methods during a single operation of a motor in order to utilize the improved operation of each method at its preferred operating point is not without challenges. Switching between two control methods will result in step or very quick changes in commanded voltage, current, or speed to the motor. Typically significant levels of current and/or torque will result in the motor as the motor attempts to respond to the new control method. Consequently, it would be desirable to provide a soft-starter with an improved transition between operating modes, to facilitate operation of the soft starter in multiple operating modes.